Thermocouple of high electromotive force



Aug. 27, 1935. V w. GOD ECKE 2,012,465

THERMOGOUPLE OF HIGH ELECTROMOTIVE FORCE Filed June 20, 1932 \NVELNTORWilliam Gb'olecke ATTO NE-Y Patented Aug. 27, 1935 THERMOCOUPLE OF HIGHELECTRO- MOTIVE FORCE Wilhelm Giideeke, Hanau-on-Main, GermanyApplication .iime 20, 1932, Serial No. 618,112 In Germany June 20.1931

Claims.

This, invention relates to thermocouples and aims at providing improvedthermocouples of high electromotive force and thus rendering possiblethe use of very cheap measuring in stru ments, which possess a lowsensitiv'eness.

So far as thermocouples having limbs made of noble metals are concerned,the Le Chatelier couple has hitherto been most generally used, thisalone having a measuring range up to 1600 C. The E, M. F. of this coupleis, however, so small that it is necessary to use very highly sensitivemeasuring instruments which are very expensive.

The E. M. F. of thermocouples made from base metal components, such forexample as constantanand chrome-nickel couples and the like, issubstantially greater, but their measuring ranges only extend up toabout 900-1000 C., so that the extent to which they can be used islimited.

For a long time, therefore, attempts have been made to findthermocouples which, like the Le Chatelier couples,..are suitable fortemperatures up to about 1600 C. but which owing to increased E. M. F.can be connected to less sensitive and, therefore, more accurate andcheaper measuring instruments, and which able measuring instruments ofsubstantially hi her total resistance to be used with particularaccurate measurements.

It has been found that couples of higher E. M. F. and substantiallyhigher heat resistance can be produced by making one limb from platinumor one of the platinum metals such as rhodium or of an alloy thereof,and the second limb from an alloy containing platinum and rhenium.

Even with rhenium contents of about 3%, such a couple has asubstantially higher E. M. F. than one with limbs containingrespectively platinum and platinum and rhodium. In general therhenium-may amount to between 3 and of the total alloy, while it mayadvantageously be about 10%. Rhenium contents outside these limits may,however, sometimes be used.

Tests have shown that in comparison with the E. M. F. of 16.6 millivoltsat 1600 C. produced by the Le Chatelier couple, the E. M. F. of a coupleaccording to the invention and having a limb containing about 10%rhenium in addition to platinum is 45 millivolts, while an increase ofthe rhenium content leads to an increase in the E. M. F. to 60millivolts.

By alloying further metals, such as rhodium, iridium, iron, nickel orcobalt, to the platinumrhenium alloy, besides increasing the sensitivityand heat-resistance, this has the further advantage that the strongtendency of the platinum metals to recrystallization can be avoided.Thus, for example, an addition of 3% rhodium is enough to prevent thealloy becoming brittle owing to recrystallization. A limit is set to theamount of further elements which may be added 5 by the resultantincrease inrthe specific resistance.

The heat resistance of the couples can be increased further if rhodiumis included in the second limb. Thus for example thermocouples in 10which one limb contains rhodium and platinum and the other platinum andrhenium are more heat-resistant than those in which one limb consists ofplatinum alone and the otherof platinum and rhenium. in

With the use of a rhodium limb besides a platinum-rhenium limb, theheat-resistance is increased still further, that is'to say up to' about1800" C., but in this case theE. M. F. is somewhat smaller and amountsto about 25 millivolts at It has further been found that the combinationof one limb made of a platinum-rhenium alloy with another limb made ofpalladium gives a thermocouple which has an E. M. F. at temperatures of1000 C., and over which lies uniformly. about 15 to 20 millivolts higherthan that of a combination consisting of platinum on the one hand andplatinum-rhenium on the other hand. The use of palladium for the'secondlimb also makes the thermocouple cheaper. because the much smallerspecific weight of palladium makes possible a. reduction in the weightof the limb in comparison with a platinum limb.

It has also been found that, in thermocouples of the-kind described sofar in this specification, the rhenium of the platinum-rhenium alloy tobe used for one limb can be replaced by metals which are adjacent torhenium in the periodic system and especially by those of the fifth and4 sixth groups of the periodic sys'iem. Thermocouples of which one limbconsists of platinum or palladium or another platinum metal while the 1and"4% vanadium, between about2an'd 10% molybdenum, and from 3 up toabout "1% manga'nese... I

.The E. M. F. of such thermocouples, when one 5 limb is made of platinumand the other of an alloy as described are as follows at 1200 C. I

Composition of the alloy E. M. F

Platinum-i17% tantalum"; .Q 23 my. Platinum+2,6% vanadium my Platinum 7%molyb' enum... 24 my. Platinum 9% tungsten- 36 my.

' enclosed in anon-metallic; preferably gas-tight,

materiaL, Examples of suitable materials are highly fire-resistantporcelain, quartz and the like.

limb instead of a'platinum limb, the thermocou-' The mkbr oxidation dueto the base metal character of the metals such inter alia' as tungsten rand molybdenum, as well as the volatility of the resultantioxides andany absorption or taking up of gas by the wires of thethermocouples, canbe avoided if that limb which consisted an alloy of platinum with thesaid easily oxidizable metals is In order to prevent the access ofoxygen to the enclosed wire; the latter may advantageously .be

' 'iused into the material. Preferably a ceramic tube is used to encasethe wire and the space within the tube around the'wire is evacuated or lpies have substantially higher E. M. F. In general the increase in E. M.F. at 1200" C., relatively to that or a thermocouple having the secondlimb of platinum, is about 15 millivolts. 4 'When a palladium limb isused it has been found to be particularl advantageous to encase itgas-tightly in a'ceramic material in the way described, above, in'order,to counteract the tendency of palladium to take-up gas on beingmaintainedior sometime at a high temperature and thus-to bring aboutvariations in the E. M. F. curve of the couple.-

Ii both limbs are encased gas-tightly, combina- 'tions such as.pa1ladium-platinum-!tungstenor 5 palladium-platinum-molybdenum haveproved to be. completely resistant.

' owing to their high a. M. n, have the substantial advantage thatcheaper-and less. sensitive measur-. ing instruments can be used forindicating the E. M.F. than has been possible hitherto with noble metalthermocouples.

The drawing shows in cross section a thermocouple according to myinvention.

Such thermocouples,

In the drawing, 2 2 represent the two limbs oi. the thermocouple, whichare encased in a covering-element 4 of refractory material such asporcelain or quartz. The covering may be closed at its lower endby asealing element 8 .0! .insulatin'gmater-ial of any suitable type.

The use of heat-resistant alloys with rhenium, which also have theproperty'of not being at-- tacked, affords in addition to the largerange over which they can be employed, thesubstantial advantage thatcheap, i. e. measuring instruments oflow sensitiveness can be. used, and

metalthermocouples What 1' claim is 1. Athermocouple one limb of whichconsists essentially of a metal of the platinum group while the'otherconsists essentially of platinum' and rhenium 2. A thermocouple one limbof which consists essentially of platinum and the other essentially ofan alloy .of platinum with rhenium.

3. A thermocouple one limb of which consists essentially ofpalladium'while the other consists essentially of platinum and rhenium,-

4. A thermocouple one limb of which consists essentially of a metal ofthe platinum group and the other essentially of an alloy of platinum.with

5. -A thermocouple one limb of which consists essentially of a metal ofthe platinum group and ,the other; essentially of an alloy of platinumcontaining about 10% of rhenium.

6. A thermocouple one limb of which consists essentially of a metal ofthe platinum group while the other consists essentially of platinum andrhenium, to which ametal selected from the group consisting of rhodium,iridium, iron,-nick el and cobalt is alloyed which is capable ofpreventing recrystallization.

7'. A thermocouple one limb oi which consists essentially of a metal oftheplatinum group while this hasnot hitherto been possible with noblerhenium containing from 3 to 15% of rhenium."

the other consists essentially of platinumand I rhenium, to which ametal selected from the group consisting. oi! iridium, iron and nickelis alloyed which is capable of preventing recrystallization.

8. A thermocouple essentially of a metal of the platinum-group while theother comprises essentially platinum, rhenum and'rhodium. 1

I 9. A thermocouple one limb "01 which consists the other'consistsessentially of platinum, rhen- .ium and substantially, 3% ofrhodium.

10. A thermocouple comprising two the one limb of which consistsessentially or a metal of the platinum group while major; portion of oneofwhich limbs consists of a platinum metal and the major portion of the.

' other of which .li'mbs consists of pi tinum and rhenium. I v v Iwmr-mmabnncxla

